Coronary heart disease (CHD) remains the leading cause of death in Western countries. In the United States 13.2 million or 4.85% of the population is affected, with 1.2 million new or recurrent attacks and around 500 thousand deaths per year (American Heart Association, Statistics for 2001). The disease is influenced by several well-established risk factors, such as age, sex, blood lipids, blood pressure, smoking, diabetes, and body mass index (BMI) as an indicator of overweight and obesity. The National Cholesterol Education Program (NCEP) Adult Treatment Panel III defines elevated plasma levels of low density lipoprotein (LDL) cholesterol (LDL-C≧160 mg/dL), and low levels of high density lipoprotein (HDL) cholesterol (HDL-C≦40 mg/dL) as independent risk factors for CHD. Many prospective epidemiological studies have indicated that a decreased HDL-C level is a significant independent risk factor for heart disease, while increased HDL-C levels≧60 mg/dL (≧1.55 mmol) have a protective role against CHD.
Nicotinic acid (Niacin), a vitamin of the B complex, is used for almost 40 years as a lipid-lowering drug with a favorable profile for all lipoprotein classes. Numerous clinical studies have shown the beneficial effects of niacin, demonstrating a reduction of coronary artery disease and overall mortality. Niacin is the most potent agent currently available to raise HDL. It has been proposed that niacin's main mode of action is through inhibition of lipolysis in the adipose tissue having as a result the reduction of free fatty acids (FFA) in plasma and liver and consequently the decreased production of very low density lipoproteins (VLDL), accounting for the reduction of total cholesterol (TC), triglycerides (TGs), and LDL-C. Due to the decreased TG rich lipoproteins levels, less modification of HDL particles occurs upon the action of cholesteryl ester transfer protein (CETP), resulting in a decreased catabolism of HDL. A direct inhibition of lipoprotein AI-HDL (LPAI-HDL) particle uptake by the liver has been also proposed, accounting for the overall HDL raising properties of niacin (Jin et al Arterioscler. Thromb. Vasc. Biol. 1997, 17, 2020-2028).
Niacin also has anti-diabetic, anti-thrombotic and anti-inflammatory properties that contribute to the overall cardioprotective effects. Through a variety of mechanisms niacin reduces thrombosis, such as the reduction of lipoprotein (a) (Lp(a)) which is a potent inhibitor of fibrinolytic activity, and it is the only currently approved drug that effectively reduces the serum levels of Lp(a) (Carlson et al. J. Intern. Med. 1989, 226, 271-6). Inflammation is a critical component of atherosclerosis, leading to recruitment of macrophages which both promote plaque development and decrease plaque stability thus increasing cardiovascular risk. Niacin has been suggested to have anti-inflammatory properties, such as the reduction of C-reactive protein (CRP) levels (Grundy et al. Arch. Intern. Med. 2002, 162, 1568-76). Several prospective studies have established a strong and direct correlation between cardiovascular risk and CRP levels, a measure of vascular inflammation. Extensive use of niacin has been hampered due to side effects, mainly intense cutaneous flushing.
Recently HM74A/HM74, a G-protein coupled receptor (GPCR), was identified as a receptor for niacin and proposed as the mediator of the niacin effects (Wise et al. J. Biol. Chem. 2003, 278 (11) 9869-9874 and Soga et al Biochem Biophys Res Commun 2003 303 (1) 364-369). In support, deletion of the PUMA-G (HM74A orthologue) in mice abrogated the niacin effects on reduction of plasma free fatty acids and triglycerides (Tunaru et al Nature Medicine 2003, (3) 352-255).